This invention relates to AC generators, specifically to the incorporation of a field system which does not require brushes or sliding contacts and which permits speed changes without changes in the AC frequency.
In conventional alternators the magnetic poles on the rotor have a fixed orientation with respect to the rotor. The fixed magnetic poles may be provided by having permanent magnets mounted on the rotor. The more common practice is, however to have field windings on the rotor which are supplied with DC current to create the magnetic poles with fixed orientation. The arrangement for providing this DC field current is called the excitation system of the generator. Typically the excitation system consists of a separate machine which is mechanically coupled to the shaft. Historically the exciter machines were DC generators. The DC output from the exciter was fed to the field windings on the rotor through brushes which make sliding contacts with slip rings mounted on the rotor shaft. Since the DC generator also has to have brushes on its commutator, such an excitation system involved the use of several brushes and rubbing contacts, resulting in the need for periodic brush replacements and related maintenance procedures and shut downs. Therefore brushless excitation systems came to be developed. In a typical brushless excitation system the exciter machine is an AC generator in which the AC windings are placed on its rotor. The AC output from these windings is rectified by rotor mounted diodes and fed directly to the field of the alternator without the need for any brushes or slip rings. The adjustment of the field flux is achieved by adjusting the field current of the exciter. This can be done from the stationary frame in both types of exciters. In the permanent magnet machine such a facility for adjustment does not exist. In all such AC generators with DC excitation the frequency of the generated AC is fixed by the rotational speed and the number of poles for which the generator is designed. Therefore, to ensure a constant frequency output, as is needed in most industrial power supply systems, it is essential to maintain the speed constant at the exact value determined by the number of poles of the machine.